Air Preheater

 Neccessary of APH:

1. It cools the exit flue gas in the heating of air supplied for the combustion of fuel. The use of hot air makes the combustion process efficient and lessening the energy loss. 

2. The application of hot air increase the theoretical flame temperature and to intensification of radient heat transfer in the furnace. As a result of the increase furnace exit temperature, gas temperature throughout the steam generation are raised above the level. 

3. Decrease the exit flue gas temperature to 135 to 180 °C, not only prevent acid corrosion but also reduce the loss of energy through stack and proper dispersion of exhaust gases from the stack to atmosphere. 

4. Fuel saving- 4% for a 110 °C rise in air temperature and above 11% for a 280 °C rise in air temperature in APH. 

APH is divided in  two types:

Recuperative	Regenerative
Heat is directly transferred from the hot gasses to the air across the heat exchanger.	Storage type heat exchanger have an energy storage medium called matrix, which is alternately exposed to hot and cold fluid.
Shell and tube type	Rotary type.

RECUPERATIVE AIR HEATER 

REGENERATIVE AIR HEATER 

APH efficiency effected by:

1. Boiler load variation => As the boiler load is increases, it simultaneously increases the volume and temperature of the flue gases. This results in higher convection heat transfer through the APH and improves the efficiency of latter. 

2. High sulphur fuel combustion => APH operates in the zone of lowest temperature of combustion product, therefore a part of its surface near the cold end (where the waste gas temperature may be 120-160 °C) will be at a temperature equal to the dew point of sulfuric acid (140°C) produced on reaction of SO3 with the moisture content in the combustion product. This corrosive acid film deposited on the surface of APH at the cold end will contribute to serve corrosion of the tube metal surface. 

Dew point corrosion in APH:

If the air heater operates at temperature below the dew point, deposits on metal surface of air heater are initiated by condensation of acid or moisture from flue gas. Degree of fauling depends upon air Heater heating element metal surface. Minimum metal temperature occurs at the cold end where as a result most fouling and corrosion occurs. 

Fire in APH:

If low load is carried for considerable period when firing on oil, a fine deposit (similar to lamp black in appearance) accumulates on the air Heater surface when temperature is low. Subsequently increase in temperature may result ignition of inflammable deposits. 

Prevention => 

• In rotary air Heater fixed spray are installed permanently inside the air Heater casing and coupled to fire mains.
• Quick release drain valve.